The present invention relates to a gas separation device for physiological fluids, particularly for cellular fluids such as blood. The present invention relates also to a fluid mixing device with gas separation.
It is known that any gas particles present in a physiological fluid, such as blood circulating in an extracorporeal circuit, must be effectively removed when the fluid is to be administered to a patient. It should be pointed out that this is because excessively large gas particles can be dangerous if they are transferred to a patient's cardiovascular system.
It is also known that some treatments require the simultaneous administration to a patient of both a physiological fluid, for example blood circulating in an extracorporeal circuit, and an additional fluid, for example an infusion or replacement liquid. However, before the two fluids, for example blood and infusion liquid, are transferred to the patient, it is necessary to remove any gas particles that may be present.
This document makes reference, without restrictive intent, to the field of machines for extracorporeal blood treatment, such as dialysis machines, in which field there is a known method of using at least one gas separation device operating in a line for the return of blood to the patient.
A gas separation device suitable for the application described above comprises, typically, a containing body providing within itself a chamber designed to be partially occupied by the blood which is to undergo the degassing operation. Suitable shaping of the chamber allows the blood to accumulate in a lower part of the chamber, thus promoting the separation of the gas bubbles. These bubbles can be removed through a service line or discharged directly to the exterior.
Normally, the pressure within the separation device is kept below atmospheric pressure, in order to promote the separation of the air bubbles.
The blood leaving the device described above then passes through an air bubble sensor which, in turn, can operate a safety clamp. The clamp is typically positioned on the line returning the blood to the patient, in order to prevent any event considered to be dangerous from being propagated into the patient's cardiovascular system.
Another gas separation device of a known type is illustrated in U.S. Pat. No. 5,707,431. This device comprises a cylindrical chamber divided radially into two parts by a filter, also cylindrical, located centrally in the chamber.
The blood inlet is located in a top part of the chamber and is directed tangentially towards the outer part of the chamber to create a vortex flow. The vortex flow of blood in the outer part of the chamber is converted to an essentially vertical flow as a result of the passage of the fluid through the cylindrical filter. The blood proceeds downwards and passes out through an aperture in the lower part of the separation chamber.
The air bubbles, which because of the vortex motion of the blood tend to move towards a perimetric area of the chamber, move upwards towards a hydrophobic membrane which is located at the top of the chamber and which allows the gas to be discharged to the external atmosphere.
Finally, a one-way valve located next to the membrane prevents the air from returning into the chamber.
The following publications:                FR 2,508,319;        EP 0 661 063;        U.S. Pat. No. 5,421,815;        JP 90-182404;        “Interaction of blood and air in venous line air trap chamber”, extract from Artificial Organs (vol. 14, suppl. 4), K. Ota and T. Agishi, ICAOT Press, Cleveland 1991, pp. 230-232; and        ASAIO Journal (1993), “Suppression of thrombin formation during hemodialysis with triglyceride”disclose the use of a layer of fluid interposed between the free surface of the blood and the air, in order to reduce the appearance of coagulation phenomena.        
In particular, EP 0 661 063 and U.S. Pat. No. 5,421,815 illustrate a blood/air separation chamber comprising a tubular containing body provided with a top cover to which a blood inlet tube is connected. In the described chamber, the blood accumulates in a lower part of the tubular body; in order to separate the blood from direct contact with the air, a static layer of anticoagulant material is used, comprising triglyceride acids and an antioxidant interposed between the free surface of the blood and the air. Since this static layer is carried on the surface and is only miscible with difficulty with the blood, it prevents direct contact between the blood and the air.
Finally, document WO 00/32104 discloses a pressure sensing system in which a service tube, partially filled with a cell-free solution, is interposed between a pressure sensor and a blood circuit. The cell-free solution creates a separating column between the blood and air which, because of the small section of the service tube, prevents or reduces to a minimum the propagation of one or more components of the blood towards the end of the service tube which is occupied by the air.
The technical solutions described above have been found to have certain aspects that could be improved.
In the first place, many of the devices mentioned have large blood-air interfaces which, as is known, promote the formation of clots and encrustations, or, alternatively, in solutions using a static layer to separate the air from the blood, require the use of chemical substances immiscible with blood which float on the surface of the blood to prevent its direct contact with the air.
In the second place, the devices mentioned are not capable of both efficiently mixing the blood with any infusion or replacement fluid that may be present and simultaneously and effectively degassing both fluids.
It should also be noted that the conventional solutions require the presence of a relatively high volume within the gas separation device; in the case of dialysis treatment, for example, the quantity of blood constantly occupying the interior of the separation device increases to a considerable and undesired extent the total amount of blood kept outside the patient.
It should also be noted that, if a tangential blood inlet is to be used to create a vortex to promote the separation of air bubbles from the blood according to the known method, it is necessary to have a central filter to prevent the transfer of the air bubbles to the exit of the separator. The presence of the filter not only increases the overall cost of the device, but also constitutes an additional element which may lead to encrustation and undesired deposits of particles, particularly when part of the filter is located in an air-blood interface area.
Moreover, the known devices which have been described are relatively unsuitable for permitting high blood flows (of the order of 500 ml/min.), low pressure drops and absence of stagnation points, with simultaneous and effective mixing of any infusion liquid.